Variable (Ramped) Flip Angle Pulses

How does varying the flip angle improve TOF MR angiography?

In 3D time-of-flight (TOF) MR angiography slabs of tissue 5-10 cm thick are typically imaged. Flow-related enhancement is highest for vessels first entering the slab. The farther into the slab they travel, the more these vessels become saturated by repetitive RF-pulses. This results in progressive loss of MRA signal from vessels extending deep into the imaging volume.

Progressive signal loss of vertebral arteries due to saturation effects as they travel more deeply into the slab

The relative saturation between flowing blood and stationary tissue can be controlled by adjusting the RF-flip angle (α). In general, higher flip angles produce greater degrees of background saturation.

Normally an RF-pulse is designed to produce a constant flip angle across its entire volume of excitation. Ramped RF-pulses are a special class of asymmetric pulses with flip angles that vary as a function of position. In those most commonly used for MRA, the flip angle is made to increase linearly along the direction of flow. For example, the flip angle might be 20° at the entry slice, 30° at mid-slab, and 40° near the exiting slice.

Ramped RF-pulse profile

Progressively higher flip angles in the deeper slices means that the stationary tissue there will become more saturated and further suppressed. Although intravascular enhancement still diminishes as the vessel travels deeper into the slab, its relative contrast compared to background tissue will be increased and the vessel will be more conspicuous. As a rough approximation, the use of ramped pulses allows slabs to be at least 1.5−2.0x thicker than they would otherwise be without such pulses.